Christopher M. Liauw

Behaviour of nanoparticle (ultrafine) titanium dioxide pigments and stabilisers on the photooxidative stability of water based acrylic and isocyanate based acrylic coatings

A series of nano-particle grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their photochemical activites have been determined and compared in water based acrylic and isocyanate acrylic coatings with typical benzophenone and hindered piperidine light stabilisers. Their performance on wood and aluminium substrates is assessed by FTIR, colour and gloss change and mass loss following artifical weathering. UV absorption analysis of benzophenone and benzotriazole chromophore based UV absorbers shows that they absorb more strongly in the near UV below 350 nm. However, nanoparticle rutile absorbs more strongly above this range and therefore, operates as a strong opacifier. Anatase also possesses opacifying behaviour, but to a lesser degree than rutile above 380 nm. In water based acrylics the absorbers undergo decomposition during irradiation whereas the nanoparticles, by virtue of their inorganic nature are inherently stable. Mass loss experiments indicated that anatase is a photosensitiser, though the intensity of the effect was found to be dependent upon the nature of the coating. Rutile was found to be an effective stabiliser with performance greater than or equal to the organic absorbers and HALS. Combinations of anatase HALS were found to be antagonistic. Little or no synergy was observed between rutile and HALS. The outstanding performance of both anatase and rutile nanoparticles is visibly and colourimetrically evident on clear acrylic wood coatings with rutile being the more effective. The 70 nm particles are more effective than the 90 nm particles. However, this data for anatase is in marked contrast to the chemical changes from FTIR analysis. Here the difference may be due to the colour bleaching effect on the lignin products through the photosensitising activity of the anatase. From a commercial point-of-view coated nanoparticles offer a significant opportunity for cost-effective benefits over conventional organic absorbers and HALS for the photoprotection of acrylic coating systems.

Ageing and stabilisation of filled polymers: an overview

The durability of filled polymers is a scientific and technological problem involving a complex interplay of phenomena and mechanisms many of which are little understood. There are a number of issues that require addressing in this regard. The first relates to the stability of the filler itself, especially if it is a coloured pigment. In this regard organic pigments may fade or change colour to an unacceptable shade. The second involves complex filler-polymer interactions and these, in turn, are further complicated by the nature of the environmental ageing conditions such as temperature, atmosphere, humidity and light source. The third problem relates to filler-stabiliser interactions where chemical and physical adsorption processes or sensitisation reactions can control the nature of both the oxidation and stabilisation processes. In this regard fillers with high surface areas and pore volumes can markedly influence such mechanisms, especially hindered phenolic antioxidants and hindered piperidine light stabilisers (HALS). This paper presents an overview of our current understanding of the complexities of filler-polymer and additive interactions with a major emphasis on carbon black, silica and titanium dioxide pigments.

Effect of silane-based filler surface treatment formulation on the interfacial properties of impact modified polypropylene/magnesium hydroxide composites

Abstract Magnesium hydroxide (Mg(OH) 2 ) flame retardant filler has been treated with combinations of vinyltriethoxysilane (VS), dicumyl peroxide (DCP) and trimethylolpropane triacrylate (TMPTA) and melt blended to V f =0.14 and 0.41, (30% w/w and 65% w/w) with an impact modified polypropylene matrix. The VS/DCP combination afforded the best balance of strength and toughness at V f =0.41. The superiority of this system was found to be due to encapsulation of the filler particles by the crosslinked elastomeric phase of the matrix which was confirmed by secant modulus measurements and low temperature fractography. The data obtained from this study form the basis of a proposed mechanism for this encapsulation effect.

Influence of ozone on styrene–ethylene–butylene–styrene (SEBS) copolymer

Non-commercial and commercial SEBS copolymer materials have been subjected to severe ozone treatment for different periods of time and concentrations in an ozone cabinet in order to obtain a deeper understanding on the mechanism of ozone attack in this type of material. The polymer materials were subsequently analysed by FTIR (ATR method), fluorescence and phosphorescence spectroscopy. Hydroperoxide analysis and determination of gel content was also carried out. Original functionalities in the SEBS based on aliphatic vinyl and aromatic (styrene) structures were observed to decrease in intensity and these were consistent with the concurrent formation of ozonide groups. Immediate exposure of SEBS to ozone resulted in the rapid and consistent formation of a variety of carbonyl and unsaturated carbonyl products based on aliphatic esters, ketones, and lactones as well as aromatic carbonyl associated with the styrene phase. These were followed by a more gradual formation of ether, hydroxyl and terminal vinyl groups with time and concentration. Also, of interest was the evident formation of a strong enol tautomer of a β-diketone functionality. These functional group changes were specific and concentrated on the very surface layer of the SEBS only. Whist there was strong evidence for hydroxyl group formation hydroperoxide analysis showed minimal evidence for active peroxides although growth was consistent with ozone dosage for the less ozone resistant materials. No crosslinking was also found in this material. Early decreases in in-chain vinyl groups by FTIR analysis were also consistent with an observed decrease in fluorescence functionalities in the SEBS associated with primarily trans-stilbene groups whereas longer periods of exposure showed new fluorescence functionalities. Phosphorescence analysis showed the formation of acetophenone end groups on the styrene chains associated with chain scission within the aliphatic rubber-styrene interphase region. Commercially ozone resistant SEBS materials were found to contain lower levels of fluorescent trans-stilbenic chromophores indicating this to be the weak link at the interphase in non-commercial ozone susceptible samples. Mechanistic routes for these processes are proposed and discussed.

Interrelationship of spectroscopic properties with the thermal and photochemical behaviour of titanium dioxide pigments in metallocene polyethylene and alkyd based paint films: micron versus nanoparticles

Abstract A series of nano and micron particle grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their thermal and photochemical activites have been determined in monomodal metallocene polyethylene (with a monomodal mass distribution) and alkyd based paint films. Their performance has been assessed by FTIR and hydroperoxide analysis, chalking, gloss change and weight loss following artifical weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, luminescence, 2-propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of charge carrier dynamics in the pigments. Photo-oxidation studies on polyethylene containing nano-particle and pigmentary grade titanium dioxide pigments show that in general the former are more photo-active with anatase and rutile forms exhibiting high activity. Pigment particle size and surface area clearly play an important role in dispersion and any polymer–pigment interactions. This relationship is also extrapolated to the thermal sensitisation activities of the pigments on processing and oven ageing. Nano-particle pigments induce oxidation of the polymer during processing and long-term thermal ageing accelerating the formation of hydroperoxide and carbonyl groups. This behaviour can play a major role in controlling the photo-activity of the pigments in a polymer matrix. Photo-oxidation studies on paint films show a clear demarcation between nano-particle and pigmentary grade titanium dioxide with the former being more active. SEM analysis shows the formation of pits and holes emanating out from the particles. Model system studies based on 2-propanol oxidation and hydroxyl analysis go some way to predicting pigment activities but clear correlations do not exist. On the other hand the real-time microwave cavity perturbation method can be used as a gauge of photo-activity in titanium dioxide pigments. This, we believe, is a useful application, which could form the basis of an effective characterisation technique for commercial systems.

Influence of processing aids on the thermal and photostabilisation of HDPE with antioxidant blends

Abstract The thermal and photostabilising performances of two commercial phenol/phosphite (Anox-Great Lakes) and three commercial phenol/phosphite/lactone (Ciba Specialty Chemicals) packages have been determined in HDPE in the absence and presence of two processing aids, calcium stearate and oleamide. Stabilities were assessed using FTIR, thermal analysis and mechanical property changes (tensile, elongation and impact). In terms of thermal stabilisation the phenol/phosphite blends are found to be superior to the Tris–phenol/phosphite/lactone blends. Solid-state (oven ageing) and melt (OIT) stabilities were found to follow a similar trend as does the yellowness index. The Anox blends were found to offer superior stability in terms of oxidative and mechanical performance. Colour inhibition was strong except in the case of calcium stearate. However, in the presence of calcium stearate strong synergism was observed with the Irganox blends and this may be associated with an exchange interaction/complexation between the calcium and the lactone/phosphite derivatives. These complexes appear to have high performance in thermal stabilization possibly through hydroperoxide decomposition and inhibition of carbonyl formation. In contrast, the effects of photostabilisation were significantly different with strong antagonistic effects in the presence of the calcium stearate due to the instabilities of any calcium complexes. Overall, the chemical and mechanical property changes followed a similar pattern.

Flow micro-calorimetry and FTIR studies on the adsorption of saturated and unsaturated carboxylic acids onto metal hydroxide flame-retardant fillers

Carboxylic acids, particularly fatty acids, are by far the most important surface modification systems for inorganic fillers used in the plastics and rubber industries. However, relatively little is understood regarding factors that affect their adsorption behaviour, including the interplay between acid molecular structure, adsorption conditions, and substrate chemistry. In this study, the adsorption (from n-heptane and toluene) of a range of saturated and unsaturated fatty acids, together with acrylic acid, onto aluminium and magnesium hydroxides has been investigated using flow micro-calorimetry and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). With C18 saturated fatty acids, the overall heat of adsorption per unit surface area of substrate was found to be affected by the structure of the alkyl tail. The orientation of the adsorbed unsaturated fatty acids was found to be affected by the number of double bonds in the molecule. Acrylic acid was found to be very strongly adsorbing and could displace isostearic acid from the filler surface. However, isostearic acid was found to adsorb on top of the layer of aluminium or magnesium acrylate. These findings have important implications particularly for mixed fatty acid treatment methods and the choice of fatty acid treatment for metal hydroxide flame-retardant fillers.

Influence of surface modification of magnesium hydroxide on the processing and mechanical properties of composites of magnesium hydroxide and an ethylene vinyl acetate copolymer

Magnesium hydroxide is of increasing interest as an environmentally friendly flame retardant filler for ethylene vinyl acetate (EVA) copolymers which are widely used in cable applications. In this paper, magnesium hydroxide (Premier Periclase DP393) was used at a flame retardant loading of 60% w/w in an EVA copolymer with a vinyl acetate content of 18% (molar). The particular modifiers used in this study were stearic acid (converted to ammonium stearate (AS)) and γ-aminopropyltriethoxysilane (APS). The effect of surface-treated magnesium hydroxide on the processing, mechanical and the flame retardant properties were determined, together with characterisation of the interaction between filler and polymer. These studies have shown that, relative to the untreated filler, use of AS coating leads to a decrease in tensile strength to a limiting value at monolayer coverage, but an increase in elongation at break, which reaches a maximum at monolayer coverage. However, the APS treatment enhanced both tensile stre...

The effect of resin/crosslinker ratio on the mechanical properties and fungal deterioration of a maxillofacial silicone elastomer

Variation of the crosslinker/resin ratio of a room temperature condensation cure maxillofacial silicone elastomer has caused considerable changes in the mechanical properties and deterioration by Candida albicans. Increasing the crosslinker/resin ratio caused a decrease in the tensile strength and stiffness of the elastomer. However, tear strength appeared to show an optimum value at the recommended crosslinker/resin ratio. These effects were due to the low molar mass silicone polymer that acts as a carrier for the actual crosslinking additive. The general decrease in mechanical properties was accompanied by an increase in the hexane extractables content and an increase in the Si–H content of the elastomer. The unbound polymer (extractable material) content of the elastomer was found to influence the colonization of the material by C. albicans. An increase in the unbound polymer content corresponded to an increasing number of hyphae and blastospores observed penetrating into the elastomer. The data obtained in this study have significant implications concerning the degree of control of elastomer formulation and the deterioration of maxillofacial appliances.

Magnesium hydroxide filled EVA: The effects of filler surface modification on the strength of filler/matrix adhesion and the consequences for composite structure and properties

The interaction of ammonium stearate (AS) and γ-aminopropyltriethoxysilane (APS) treatments with a magnesium hydroxide flame retardant filler and their effects on its use in an ethylene vinyl acetate copolymer compound have been investigated. The work has shown clear evidence of changes in the structure of the surface layers formed on the filler as the amount of additive is increased and the levels at which these occur can be correlated with theoretical monolayer quantities. Infrared (IR) spectroscopy data suggest that the stearate coating changes from a half salt to a full salt as the coating level is increased and that the APS coating on the filler initially has a significant bicarbonate content, presumably due to reaction with atmospheric carbon dioxide. The effect of coating level on the melt flow rate, insoluble matrix content, crystallisation behaviour, tensile properties, limiting oxygen index, and ageing of the filled compound has been studied. Distinct trends have been observed, many of which can be correlated with the structure of the filler surface layers. Of particular importance is the observation that, unlike APS, excess stearate appears to promote detrimental ageing effects.